Laser interference lithography technology is a low-cost manufacturing technique to produce a periodic structure, which mainly uses two coherent lights to overlap in space to produce periodic interference fringes. When a photosensitive material (such as a photoresist) is placed under the interference fringes, the interference fringes can be transferred to the photoresist layer to form a periodic grating structure, and then this grating structure is transferred to different materials by various etching techniques. Advantages of laser interference lithography technology are that it is low-cost and fast, and now widely used in the production of the grating structure definition of a single-wavelength distributed-feedback semiconductor laser for fiber-optic communication. The required grating periodicity is as low as 200 nanometers (nm), and the desired exposure area in wafer-level semiconductor lasers of III-V substrate is at least 2 inches. In addition, the laser interference lithography technology has potential applications including the production of a patterned sapphire substrate for high-efficiency light-emitting diodes, and the accomplishment of the wire grid polarizers, which has the common requirements toward a short-period and a large exposure area.
The most common laser interference lithography system uses Lloyd's mirror to achieve two-beam interference. This architecture comprises a mirror and a sample carrier which are arranged and fixed at right angles, and a coherent light is expanded to irradiate the mirror and the sample carrier at the same time, the light on the mirror will be reflected to the sample carrier to form two-beam interference. The advantages of this architecture are that it is simple and easy to set up, as well as that it has less impact from its surroundings, but its drawback is that it is unable to produce a large-area and uniform periodic structure. The main reason is that the two beams generated by this architecture are not evenly distributed, and they are respectively the left and right-half of a Gaussian distribution. When the two beams are overlapping in space, the sample close to the mirror is irradiated by the strong energy, and the sample away from the mirror is irradiated by the weak energy, thus a grating structure is not uniformly formed on the sample, and the uniform exposure area is generally less than 2 inches. In addition, the configuration of Lloyd's mirror interferometer is not suitable for forming long-period grating structures over a large sample area. For a grating period greater than 500 nm, this system allows less than 1 inch illumination coverage that limits its applications.
If wafer-scale production of periodic grating is desired, two independent light beams are generally used to perform the interference to improve the uniformity of the light field and to extend the achievable grating period in the Lloyd's mirror system. That is, a beam splitter is used for dividing a laser beam into two beams and then the two beams are respectively expanded, and finally two mirrors are respectively used to allow the two beams to form the interference fringes. This two-beam laser interference lithography allows the sample to be centered on the intensity maximum of Gaussian beam profile, and therefore to intercept uniform and higher illumination intensity. However, this approach lacks the simplicity for tuning the grating periodicity due to the fixed optical configuration such that this system requires laborious optical path reconfiguration for varying grating periods. Additionally, a large optical table is typically required to guarantee the necessary degrees of freedom for optical path realignment.
Referring to U.S. Pat. No. 8,681,315 published in 2011, W. Mao at al. disclosed a new type of two-beam interference lithography system with adjustable mirror planes, which has two additional mirrors to modify the travelling path of the two expanded beams, and further to freely adjust the period of the interference fringes without optical path reconfiguration. However, the system is still unfavorable to produce long-period grating that requires a small incident angle of laser beams for interference. The size of interference lithography system will be very large in order to place the sample at the interference position where two beams are overlapping. Due to the long distance between the sample and reflecting mirrors, the laser energy cannot be efficiently utilized and the exposure time will be tremendously increased.
It is therefore necessary to provide an interference lithography device, in order to solve the problems existing in the conventional technology as described above.